1. Technical Field
This invention relates generally to a scanning device, and more specifically, to a narrow field of view scanning device incorporating two multi-faceted polygon scanning wheels.
2. Discussion
Certain image acquisition systems, such as an integrated forward looking infrared (FLIR) laser radar imaging system, develop an image by collecting an incident radiation beam from a particular scene. To develop the image of the scene, a means to repeatedly scan the scene is included such that the image is developed by a line by line acquisition. It may be desirable to scan a small area, such as a target location, of a larger scene to isolate or detail certain areas of the scene. In view of this, scene images having widely varying sizes are desirable. The size or magnification of the image, defined as the field of view, therefore is varied from one application to another. Consequently, certain design requirements for scanning which acquires a small field of view are different than the requirements for other, larger fields of view.
Most prior art scanning systems incorporate a multi-faceted polygon scanning wheel which rotates at an appropriate rate to scan, and thus, develop an image of a particular scene in a line by line fashion. As the scanning wheel is rotated, a light beam directed onto the multi-faceted wheel is scanned across the scene and a return beam reflected from the scene is directed back through the system. The return beam from the scene impinges on a facet of the multi-faceted wheel through an appropriately configured aperture, and the beam is reflected off of the facet and directed to other appropriate system components to develop the image.
Each time an edge portion between one facet of the multi-faceted scanning wheel and an adjoining facet is rotated across the incident beam from a particular area of the scene, the image is not being fully developed since the full beam needs to be in contact with the facet to develop a clean image. In other words, the image quickly deteriorates when the beam hits the edge of a facet. Since the size of a facet of a multi-faceted wheel determines the size of the field of view, the more facets a wheel has the narrower the field of view of the scanning system. Accordingly, the more facets there are the more times the beam will impinge on the edge portions between adjoining facets. Since this represents downtime of the image acquisition, narrow field of view scanners are incorporated at the expense of time of acquisition.
Servo-driven oscillating mirrors offer an alternative to the multi-faceted scanning wheel. In an oscillating mirror a single faced mirror is oscillated at a predetermined rate such that the desired image is scanned accordingly. The degree of oscillation determines the field of view. However, certain alignment and physical requirements provide that an oscillating mirror cannot scan a particular analogous scene as fast as a multi-faceted scanning wheel can.
What is needed then is a narrow field of view scanning wheel which does not suffer the drawbacks of the downtime of the prior art multi-faceted scanning wheels. Accordingly, it is an object of the present invention to provide such a scanning wheel.